These days X-ray diagnosis apparatuses use as their X ray detectors an X ray flat panel detector. The X ray flat panel detector is constituted by a scintillator (for example, made of cesium iodide (CsI)) which converts X ray transmitted through an object to be examined into light, photodiodes (for example, made by amorphous silicon (a-Si)) which converts the light output from the scintillator into electric charge and switching elements such as, for example, thin film transistors (TFT) which read the electric charge, and, for example, has an active area of 30×40 cm2 and 7,400,000 pixels.
However, as referred to, for example, in R. L. Weisfield et al. “High Performance Amorphous Silicon Image Sensor for X-ray Diagnostic Medical Imaging Application” (SPIE vol. 3659, Medical Imaging 1999, part of the SPIE Conference on Physics of Medical Imaging, San Diego, Calif., February 1999 pp 307-317), since the photo diodes in such X ray flat panel detector are not saturated at the initial stage of the image taking, the output thereof shows lower in comparison with when the photo diodes are saturated. For this reason when the imaging is performed successively from the initial stage thereof, the display brightness shows dark at the initial stage and gradually becomes bright.
When an angiographic detection is performed with such X-ray diagnosis apparatus, at the same time when contrast medium is injected to the object X ray is for the first time irradiated to the object, then X ray is successively irradiated to the object with a predetermined interval and the image data at respective moments are successively output from the X ray detector. In this instance, by making use of a first image data which is output first as a mask image and by subtracting from image date after second ones, only a blood vessel image where the contrast medium is injected is extracted and such as constriction of blood vessel is diagnosed from the image.
Namely, for the same X ray dose, the following tendency is observed that the signal value of the image data taken first which is used as the-mask image shows the lowest value and the signal values taken second, third . . . , which are used as live images successively increase toward the saturation. However, in the field of X ray detection, it is not preferable in view of X ray exposure of an object to irradiate X ray to the object prior to image taking irradiation.
FIG. 5 shows a build up output characteristic diagram of an X ray flat panel detector wherein the abscissa is image number according to output order and the ordinate is relative height of the output signal. In FIG. 5, symbols corresponding to letters a˜g show different X ray doses (a>b>c>d>e>f>g) irradiated to the same X ray flat panel detector. The image number according to output order shows image taking order by the same X ray flat panel detector, in that first image, second image, third image, . . . , tenth image.
Since the X ray flat panel detector shows the build up output characteristic as shown in FIG. 5, the output values in the regions other than the contrast medium of the image data taken first which is used as the mask image and of the image data taken second and thereafter which are used as live images are different, therefore, even if the subtraction processing is performed, the regions other that the contrast medium can not sometimes be sufficiently subtracted, as a result, difference in brightness level of the background portion in the subtraction images which are used for the angiographic detection occurs and the contrast of the subtraction images is reduced, which causes a problem.
An object of the present invention is to provide an X-ray diagnosis apparatus which eliminates influences of inherent low output characteristic, namely, delay in output build up of an X ray flat panel detector in the X-ray diagnosis apparatus and permits to obtain subtraction images, namely, angiographic images of desirable contrast.
Another object of the present invention is to provide an X-ray diagnosis apparatus which eliminates influences of inherent low output characteristic of an X ray flat panel detector in the X-ray diagnosis apparatus and permits to obtain X ray images of uniform brightness which facilitates comparison observation.